Final report ARID - European Space Agencyeea.spaceflight.esa.int/attachments/parabolicflights/ID4ff6b0fa12a4… · particles in microgravity above the Netherlands, with the rising

Fly Your Thesis! 2010 – Final Report

Fly Your Thesis! 2010

Final Report ARID

Note to the reader: data presented in this document cannot be used for official peer-reviewed or popular scientific purposes (all media) without the explicit approval and consent of the authors prepared by/préparé par S.J. de Vet & L.L. Mulder reference/réference FNWI / IBED / ESS / ARID, FYT2010 Final Report issue/édition Error! Unknown document property name. revision/révision 1 date of issue/date d’édition February 28th 2012 status/état Final Document type/type de document Technical descriptionError! Unknown document property

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Fly Your Thesis! 2010 – Final Report ARID

1. Executive Summary Water repellency is a very common problem for soils in nature. The view is held that organic compounds of plants and fungi coat sand particles with a minute layer of water repellent organic molecules. Little is, however, known about how these large scale properties of water repellency are reflected at the intermediate scale of individual sand particles despite the importance of it as this scale ties the smallest scale of molecules to the large scale processes observed in landscapes. The ARID experiment carried out during the 54th ESA parabolic flight campaign is an innovative approach to studying the properties of the coating orientation at the level of an individual sand particle. In the experiment the hypothesized dynamic orientation behaviour is studied using collisions between individual sand particles. Interactions during these collisions caused by the presence or absence of electrostatics forces polar functional groups will cause a distinct dissipation in energy which can be quantifiable in terms of velocities before and after collisions. Dune sands from the Amsterdam Drink Water Supply Dunes, a well-studied area, were used in the ARID project. By varying sand samples in groups from 100-500μm and sample location (different vegetation types) initial velocities were varied in the range of 5-50mm/s. With high-speed 500fps stereoscopic imaging of the test tubes with the sand, the collisions will eventually be reconstructed in 3D to yield to best accurate results. For this effort, a special hardware set-up was designed and constructed at the University of Amsterdam for the parabolic flights, making use of the ESA build TERAS camera system. In order to understand and be able to explain the behaviour of the sample material in microgravity, the baseline data collection (BDC) aimed at, and included extensive measurements of various chemical properties and physical properties of the sand. These properties control the interactions during the collisions. Results are presented of the various techniques which have been used during the BDC to understand the chemical composition of water repellent organic material in the field site samples. These include Density Fractionation, CNS to measure the mass percentage of carbon by means of combustion and measurements using gas chromatography - mass spectroscopy (GC-MS) in combination with pyrolysis to study molecules in the soils on the basis of e.g. polarity. Physical properties of the water repellent sand particles were obtained using Sessile Drop Tests (SDT), Water droplet penetration time (WDTP) tests and Molarity of Ethanol Droplets (MED). With these results we show the well-defined nature of the samples for the experiment. We can infer how the organic carbon adhered to the grains is limited to the intermediate size fractions (106-600μm) and which contributes the most to the repellency for this size range in reference to other forms of organic material in the soil. The used particle sizes in the flight experiment provide a suitable basis for testing the hypothesis of the dynamics behaviour of the coating state. With measurements of the a-polar/polar ratios some first predictions can be made for sample groups used in the experiment on the basis of their aptitude for electrostatic interactions and dissipation of energy during collisions in microgravity. These conditions set a well-defined stage that allows the eventual comparison between hydrophobic and hydrophilic conditions, with crystalline samples as a reference. As such the properties of the collisions will allow the linking of the small scale chemistry to the behaviour of the coating and the observed effects are a larger physical scale of multiple sand particles, once these flight data are fully processed. Front page: ARID’s mission patch ‘Advancing Repellency and Innovating Detection’ showing the collision of two sand particles in microgravity above the Netherlands, with the rising sun above the horizon. The colours of the European map are derived from a desertification map where the effects of water repellency are the strongest within Europe.


2. Student Team Description The ARID team consists of PhD students Sebastiaan de Vet (UvA), Lieke Mulder (NIOZ) (both MSc students at the time of the start of the project) and is supported by Christina Giannopapa (TUe) and the endorsing professor Erik Cammeraat (UvA). Students: Sebastiaan de Vet, MSc is a PhD candidate to the Earth Surface Science group, Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam (UvA), the home institution of the ARID project. He graduated from his master Earth Science at the UvA and specializes in planetary geomorphology. For his short-term PhD he currently studies the role of planetary variables in landscape processes using Icelandic landforms for understanding analogues at the surface of Mars. He works on experiments with wind-induced particle detachment at various atmospheric pressures inside a hypobaric wind tunnel and also studies the influence of gravity in the granular behaviour of avalanching sediment. For ARID he was involved as the Experiment Principal Coordinator (EPC) and was primarily in charge of the overall management of the project. In addition he was the lead-engineer for the design, construction and integration of the hardware, in close consultation with the engineers and experts of external (industrial) subcontractors. In preparation and support of the ARID parabolic flight project he co-supervised several BSc theses on the topic of soil water repellency.

■ More information can be found at: http://home.medewerker.uva.nl/s.j.devet Lieke Mulder, MSc Lieke Mulder is a PhD student at the Royal Netherlands Institute for Sea Research (Royal NIOZ). Her research focuses on the role of microphytobenthos (microalgae present in subsurface sediments of intertidal flats) on the phosphorus (P) dynamics of the western Wadden Sea. Effective wastewater management has caused reduction of riverine discharge of P in the Wadden Sea since the 1980s. It is hypothesized that sediments of the shallow Wadden Sea function as a buffer, potentially realising P to the overlying water. Benthic microalgae are located close to the sediment P source and by uptake and biogeochemical alteration of their surroundings acting as a filter between sediment and water column. Understanding P exchange between sediment and water column and the benthic-pelagic coupling of primary producers will help us understand the food web of the western Wadden Sea as a whole. She graduated from University of Amsterdam in Earth Sciences, specializing in the relationship between nutrient dynamics and grass-encroachment in dune areas, and also focusing on the role of phosphate. For her masters she worked in the same dune area where the ARID samples were taken for the Fly Your Thesis campaign. Within the Fly Your Thesis project she was responsible for sample collection, lab experiments and up-scaling to the natural environment.

■ More information can be found at: http://www.nioz.nl/nioz_nl/ffe83eab8b26198d9ea4c98d0ffbd62c.php


Scientific staff members: dr. Christina Giannopapa is assistant-professor multiphysics phenomena at the department of ‘Mathematics & Computer Science’ of the Technical University of Eindhoven. Her research interests and background varies from the physics of blood flow to food processing, but they are all centred around the interaction between fluids and structures. She is currently foremost expert on the operation of the TERAS camera and data acquisition system used in the ARID experiment and gained valuable experience for the ARID project by working with the TERAS system on airborne ATV and Hayabusa re-entry observation campaigns. As assistant-professor in Eindhoven, she will interface with PhD students and MSc students for the image processing of the imagery acquired during the parabolic flight campaign.

■ More information can be found at: http://www.espi.or.at/index.php?option=com_content&task=view&id=442&Itemid=1 dr. Erik Cammeraat is the endorsing professor of the ARID project. He is appointed as Associate Professor within the research group ‘Earth Surface Science’ of the Institute of Biodiversity and Ecosystem Dynamics (IBED) and is appointed too as the director of the educational programmes of the BSc & MSc Earth Sciences. His research interests are related to geomorphology and land degradation processes with a particular interest in 1) the interaction of physical, chemical or biological processes in landscape processes and landscape evolution, 2) the development of emergent landscape properties as a result from the interaction between different processes at different scales, 3) the role and fate of organic carbon in erosion and sedimentation and 4) comparative geomorphology using terrestrial geomorphology to understand analogue landscapes on Mars. He is involved in teaching in both the curricula of the BSc and MSc Earth Sciences as well as in the BSc Future Planet Studies and incidentally in the BSc Biology, and the MSc Forensic Sciences. As Joint Editor of CATENA, an interdisciplinary journal of Soil Science-Hydrology-Geomorphology, he focuses on Geo-ecology and Landscape Evolution.

■ More information can be found at: http://home.medewerker.uva.nl/l.h.cammeraat dr. Thodoris Karapansios was as ELGRA mentor involved during the initial definition phase and proposal writing of the project.

■ More information can be found at: http://users.auth.gr/~karapant/tdk.html


Figure 1 The ARID team at the Bordeaux airstrip during the 54th ESA parabolic flight campaign in May 2011. From left to right: Lieke Mulder, endorsing professor Erik Cammeraat, Christina Giannopapa and Sebastiaan de Vet.


3. Project Description

3.1. Scientific objectives

Ever returned from your holiday and found out that your plants have been deprived of water for too long? In a last desperate effort to save their lives you might consider pouring some water on the soil, only to find out the water is not absorbed and the majority probably ends up next to the pot. This form of water repellency is a very common problem for soils in nature. Although it is inconvenient for your plants at most, the effects on landscape scales can be devastating. The common view is held that organic compounds of plants and fungi coat sand particles with a minute layer of organic molecules. Adding warmth or drought stress in the form of e.g. forest fires, poor land management or effects resulting from climate change allows this coating to become a water repelling layer. Soil scientists understand these effects very well on the scales of landscapes, sand heaps and to some extend of the involved chemistry (figure 2). However, little is known about how the large scale properties of water repellency are reflected at the intermediate scale of individual sand particles. This scale has often been difficult to measure yet it is possibly the most important, as it ties the smallest scales of molecules to the large scale processes observed in landscapes. The aim of the ARID experiment is to advance water repellency research within this niche by studying particle-scale interactions in microgravity.

Figure 2 An example of water repellent sand in the Dutch dunes of North-Holland. Despite abundant water, no droplet infiltrates the soil to alleviate the drought stress of the surrounding plants. On sloping terrain, these effects lead to increased erosion and natural hazards such as mud-flows and localised erosion. The water repellent coating of a sand grain consist of amphiphilic molecules; organic residues from plants and fungi which poses a charged polar head, and an uncharged a-polar tail. In moist soil conditions, the coatings will have the polar heads sticking out and water can easily adhere to these surfaces. Under drought stress a remarkable process occurs as the soil particles become water repellent when the molecular coating ‘flips’ and the a-polar tails are oriented to the exterior. The same soil particles can thus have two totally different states, depending on the environmental


conditions. Although never measured directly, is has been assumed for decades that this dynamic process is causing water repellency. The ARID experiment is an innovative approach to studying the properties of the coating orientation at the level of an individual sand particle. In the experiment the hypothesised orientation and associated charge properties of the coating’s state are utilised by studying collisions between individual sand particles. When charged (non-water repellent) particles collide, they will slightly repel each other and these collisions thus differ from those with ‘neutral’ grains (water repellent or stripped from their coating). The only suitable research environment for observing such collisions, free of interfering processes, is in microgravity. Kinetics of these collisions are used to make the link between the chemical and larger scale (physical) soil properties. The collisions of the sand particles in the experiment are started by giving the test tubes a starting velocity using a special launch platform. Sand and sand-analogue particles with a normal coating, water repellent coating and stripped from a coating will be used and cover different particles sizes and vegetation. Their water repellent properties are measured pre-flight under a macroscope to measure the contact angle with a water droplet. Once the particles are free-floating and colliding in microgravity, three high-speed camera’s will record the collisions with high precision in 3D. Post-flight data processing then allows the use of the microgravity data as a bridge, linking chemical and field studies in one broad approach to studying the mechanisms of soil water repellency.

3.2. Experimental set-up The ARID flight frame (figure 3, affectionately called the ‘Mammoth’ by some, for its sheer size and weight) consists of a payload and bus section. The functional block diagram in figure 4 gives an overview of the various components and implemented systems in the experiment. A sample platform is placed inside a polycarbonate box and is used to subject samples to a velocity in order to suspend particles in the air inside the containers and give them an initial velocity. The second part of the experiment frame covers the TERAS data acquisition system (Greek for ‘monster’) that is provided for this experiment in cooperation with TUe/ESA. The payload section also houses the sample storage drawer. This drawer accommodates the storage of the sample containers. The bus section consists of the electrical infrastructure, wiring of the flight frame and comprises of various switches, emergency stop-button, fuse box and the ground fault interrupter. These different parts of the system are described in greater (engineering) detail in the proceeding paragraphs of this section. For more detailed information and a comprehensive overview of the set-up we refer to the Experiment Safety Data Package (ESDP, version April 23rd 2011) of the experiment.

Figure 3 Engineering drawing of the flight frame and a photo of the fully assembled ARID flight frame at the Soil Micromorphology Lab (SML) of the University of Amsterdam, The Netherlands. The base bars used for attaching the structure to the aircraft seat rails are not attached in the configuration of the right-hand photo.


3.2.1 Linear Motion System (LMS) The linear motion system is the central ‘business-end’ in the experiment set-up and consists of a sample platform mounted on a vertically oriented (along z-axis) linear guide. The system is controlled via a stepper motor controller and a Human-Machine-Interface (HMI). The motion platform is designed to be moved up and down along this z-axis of the experiment in order to accelerate the sand particles in our samples to an initial velocity and effectively suspend them in the medium. This facilitates the recording of collisions between individual particles. 3.2.1.1 Linear drive The linear drive system used in the experiment is an off-the-shelf PAS42BR linear guide from Schneider Electric, designed and constructed by the company Electro ABI. The system consists of a roller guide system with integrated railing. The 206mm long slide on the linear guide is driven using a toothed belt that runs around the two driving heads on either side of the linear guide assembly. A stroke length of 250mm is required in our case along the direction of motion (2x the length of a sample container in a +z or –z direction and a 2,5 cm margin on both sides).

Figure 4 Linear guide during hardware integration (left), after attachment of the sample platform and the cameras (centre) and fully assembled with the light shroud placed on top (right). 3.2.1.2 Stepper motor and stepper motor control A Schneider Electric stepper motor drive system consisting of an SD3 stepper motor drive and a BRS3 3-phase stepper motor. In our set-up, the linear guide is driven by a Lexum DS3 series BRS368 3-phase stepper motor (an industrial-grade stepper motor running on 230 VAC) via a 20:1 planetary gearbox which is mounted at the top of the linear guide. The torque of the stepper motor while powered is sufficient to hold the sample platform stationary during a 2g pull-in. In addition, the stepper motor controller has an output for a direct connection of a holding brake that allows the brake to lock the platform when during take-off and landing, and when the unit is powered down (e.g. during an emergency stop or when the safe mode function is activated). The holding brake is an electromagnetic spring force brake and fixes the motor shaft after the motor current is switched off (e.g. in case of power interruption or emergency stop). The motor shaft must be fixed if torque loads resulting from weights act on it, e.g. in the case of Z-axes in systems such as in our experiment. Using limiter sensors we apply an absolute positioning of the system by incorporating the maximum position of the system in both the software and by a hardware limit. Prior to use, the system first needs to be ‘homed’. During the homing process, a reference movement is carried out where the motor moves to a defined position of the lower sensor on the axis. The position is defined with a mechanical switch in the form of two end-switch limiters (PNP-NC).


The motor controller has to receive directions and its settings from an external source and is hence controlled by the operator using a human-machine interface (HMI) with an integrated stepper motor control card (PLC). In our set-up we use the 5.7” Berghof DC1005T-T human-machine-interface, which is a colour touch screen system. The man-machine-interface allows the setting a predetermined value for speed (varying between 5-50 mm/s) and is activated by the selection by the direction of movement (+z or –z), depending on the dynamic conditions of the sand moments after the parabola insertion.

3.2.2 Configuration of equipment on the sample platform The hereafter called ‘sample platform’ is constructed from a 13mm thick 450x300mm aluminium bench plate from Edmund Optics. The platform allocates three sample containers and two high speed cameras. We use two high-speed cameras of the Basler A504 type to record the collisions of particles inside the three sample containers. Nikon Nikkor 28mm F3,5 lenses with macro converter are used in combination with the Basler cameras to obtain a stereoscopic image of the sample tubes. This configuration provides the possibility of creating 3D reconstructions of the collisions.

Figure 5 Photo’s of the used camera, a Basler A504 and the Nikon Nikkor 28mm F3.5 lenses attached to the Basler cameras. Right image showing the location of the Basler camera’s and their lenses (left), temperature probe and the small 3 axis accelerometer (samples shown during a calibration setting, not representing experiment lay-out). The Basler A504 cameras are slightly offset by an angle of a few degrees to create a stereoscopic set-up for 3D particle image velocimetry. These three cameras are rigidly fastened to the platform by attaching them to a stainless steel 0.5” diameter 2” long camera post and a clamp-ring. These posts are attached using a post holder with an M6 bolt to the sample platform. As temperature is an important parameter which affects the repellent properties of soil particle, temperature conditions during the flight were recorded as part of the Baseline Data Collection. On top of the platform a Veriteq EPT-4000 temperature probe was hence mounted and used for measuring the ambient temperature inside the shroud during the entire flight. This unit is connected to a high-accuracy space-qualified Veriteq SP4000 data logger (originally produced by Veriteq for flight on 8S DELTA mission to ISS) that will be mounted on the backside of the aluminium plate that fixes the screen to the frame. In addition, a Brüel & Kjær miniature Triaxial DeltaTron type 4525-B-01 accelerometer was mounted to the platform too, to measure and record 3-axis vibration data experienced by the samples during the flight (figure 6). The accelerometer is connected to a Brüel & Kjær LAN-XI data acquisition unit and thus functions as a stand-alone unit, not drawing power from the A/C.


Finally, a black shroud made from ESD-proof plastic (PP) serves as a light-trap and is placed over the sample containers to prevent nuisance of cabin lights in the recording set-up (figure 7). Such glares on the exterior surface of the sample containers would hamper the recordings. The sample containers are lit from above to create high contrast imagery using two white LED strips. These integrated units with 24 LED total on 12V will, in combination with the black backdrop of the shroud create the proper contrast for the recordings. The shroud itself is attached to the platform using 4 ball-lock pins (5mm diameter, 10mm length) with aluminium brackets (Bosch 25x25). When servicing the platform the operator(s) were electrically grounded in order to remove static electricity in the system that may hamper the experiments.

A B C

Figure 6 The Brüel & Kjær Triaxial DeltaTron type 4545-series accelerometer, roughly 1cm3 in size (A) and standalone LAN-XI data acquisition unit (B) used in our experimental set-up. On the right (C), showing the battery pack.

Figure 7 Image showing the ESD-proof shroud’s exterior (left). Centre image shows the shroud interior with the LED-strips and wiring. Left, the type of ball-lock pin used to attach the shroud to the platform. 3.2.3 Sample containers, platform and in-flight storage Confinement of the sample material was important for two reasons, one was not to contaminate the cabin environment with loose granular particles that could hamper the respiratory track, and two, for maintaining a closed environment where the coating state was not altered by changes in relative humidity. Sample containers used to confine the sample material are constructed from off-the-shelf polycarbonate Oak Ridge centrifuge containers from Nalgene with a volume of 30ml. The tubes are attached to the platform using a 30mm diameter post holder from Edmund optics. The tubes are inserted in these post holders which have been milled out to an inner diameter from 25,5mm. A screw clamps the tubes in the holder at the cap while a rubber o-ring around the container assures the proper and fixed insertion depth. Each of the three post holders is attached to the aluminium


attachment base using an M16-M6 adapter. These adapters screw onto the underside of the post holder and fit the M6 threaded holes of the attachment strip (figure 8). The aluminium attachment strip is then fitted to the platform using two M6 bolts.

Figure 8 The polycarbonate Nalgene container is shown right. The modified post holders used to attach the sample containers to the sample platform (centre) and an overview of the sample container holders inserted with the cap into the post holders (right). Sample containers were stored inside a sample drawer during the flight. A double containment level during the microgravity phase in the storage drawer was achieved by storing the sample containers inside the drawer inside zip-lock-bags. In order to prevent static electricity build up on the sample container that influences the experiment, the containers were packed inside special ESD (electro-static discharge preventing) type zip lock bags. Transfer of the sample containers from the storage drawer to the polycarbonate enclosure is facilitated using the shroud of the sample platform. The inside of the drawer is padded with foam to reduce the amount of vibrations experienced by the samples during storage. A one millimetre thick polycarbonate plate was installed to prevent items from floating into the cabin if e.g. the drawer would be unintentionally left open or otherwise unlocked during a microgravity phase.

3.2.4 TERAS camera and data acquisition system The ESA-developed TERAS multi camera system was used for the data acquisition and storage of the video signal. TERAS was originally developed for use on parabolic flights and consists of two high speed cameras used in the experiment subsystem, writing CCD frames to the rack-mounted data acquisition system using an IO Industries DVR Express CLFC frame grabber. The system is based on 3 DVR Express CLFC boards. Two CLFC-full boards - complete single PCI card solution to provide real time video recording to hard disk for high-speed cameras. Each board has 12 disk drives (4 channels of 3 drives). One CLFC-Dual Base board - versatile card that can simultaneously record from 2 Camera Link Base cameras and write the data directly to Fibre Channel drives). The board has 6 drives (2 channels of 3 drives). Finally, three drives on one channel enable the system to record at 180 MBps sustained on each channel (maximum possible bandwidth). If fine, with 10 channels and 30 drives, the channel can go up to 1800 MBps. The computer system comprises a main PC unit with several hard disks (mounted in two additional PC-sized racks) for data acquisition and storage. The system is able to simultaneously record quantitative image data of different pixel format (512x512 to 4kx4k), pixel depth (8 bit to 24 bit) and frame rate from different electronic cameras (CCD, C-MOS, etc.) with different interfaces (RS-, LVDS, custom, etc). We use the system for recording with 2 high speed cameras continuously for 20 seconds during each parabolic manoeuvre, at max. 500 fps via the DVR Express CLFC frame


grabbers. System interface connections are CameraLink Base, Medium, Full. The type of connectors include: 4 for the CameraLink Base (2.04 Gbps each) or 1 for CameraLink Medium (4,08 Gbps) + 2 for CameraLink Base (2.04 Gbps each) or 2 connectors for CameraLink Full (5.44/6.80 Gbps each) or 2 connectors for CameraLink Base (2.04 Gbps each) + 2 connectors for CameraLink Full (5.44/6.80 Gbps each).

3.3. Experimental procedure This sections sets out to describe the procedures for the baseline data collection (pre and post flight characterisation of samples used in the experiment). Figure 9 illustrates the general workflow of the project. This section will focus in greater depth on the data collection of the Baseline Data Collection (BDC) phase used to determine the physical and chemical characteristics of the sample material used in the experiment, and which were carried out especially for the preparation and support of the ARID project, whereas section 5 will focus on the data processing of the parabolic flight experiments and the relation of these experiment to the BDC.

Figure 9 Concept block diagram showing the global workflow of the ARID project.

3.3.1 Baseline Data Collection (BDC) The baseline data collection is a crucial part of the ARID programme and started before the parabolic flight campaign and ran almost parallel to the construction of the hardware and parts of it are continuing at the writing of this document. These studies involved the quantification of various key soil parameters that contribute to water repellency. They include extensive measurements of various chemical and physical soil properties. These properties are relevant to understand in greater detail as these controls the collisions that were eventually studied in the parabolic flight experiment and help to understand the properties of coatings of individual sand particles. Hence great care is given to the characterisation and quantification of these properties. A major contribution to the BDC has been given by three bachelor students who prepared several studies into the soil characteristics and both chemical and physical water repellent characteristics of the area from which the flight samples were selected. 3.3.1.1 Field locations and field analogue samples


Dune sands from the Amsterdam Drink Water Supply Dunes (Map A) were used in the ARID project. Water repellency in the Dutch coastal areas has been extensively studied by other research groups in the Netherlands (e.g. Dekker, Ritsema en Oostindie, 1999) and thus these areas provide a well defined cadre for studying water repellency and its relation to pre-existing studies in these areas. Sites with similar vegetation types were chosen to sample in the Hinter Dunes and Young Dunes. For the sampling the following strategy was used. Within plots of 10x10 meters, several 30x30cm subplots were selected to determine vegetation cover and for sampling of the upper 5 centimetres of the topsoil as these layers are mostly found to be water repellent (Dekker, Ritsema & Oostindie, 1999). Prior to further analyses these samples have been homogenized to compensate for sorting effects during transport and to create a spatially averaged value for the water repellency. An inventory of local vegetation shows the following species to be dominant and they are given in their native Dutch names (with the Latin name in between brackets). Important species in the area are ‘Buntgras’ (Corynephorus canescens), ‘Haarmos’ (Polytrichum juniperinum) and these already related in literature to the water repellency of sandy dune soils (Neinhuis & Barthlott, 1997 en Spröte et al., 2009). Table 1 overview of vegetation present in the Dutch dunes were water repellency is present in the upper 5 cm of the topsoil (data: Jansen, 2010 and De Graaf, 2010).

[ Confidential ] In the flight experiment two types of analogue particle are also used in the form of spheruled and natural quartz particles which have been coated with organic compounds that render these particles water repellent. These have been produced specially for the ARID experiment by the Institute of Soil Science of the University of Hannover (Germany). These coatings are artificially created; they do not possess the capability of alternating between hydrophobic and hydrophilic states. These particle do poses the property to maintain their properties and their morphology makes them ideal as a reference for the less well-rounded sand particles (e.g. as a reference for the image and collision reconstructions). As such non-coated and coated silica particles of various sizes were used.

[ Confidential ] Map A topographic map showing of the sample sites that were explored during the BDC of the ARID project and the associated BSc theses field studies inside the ‘Amsterdamse Waterleiding Duinen (AWD) in the province of North Holland. West: sample plots in Young Dunes, East: sample plots in Hinter Dunes. Eventually, only samples were flown from the BDC sites, whereas the additional flight Sample Sites were excluded. (copyright © Dienst voor het kadaster en openbare registers, Apeldoorn)


Figure 10 Support students at work on reference site 1 with ARID team member Lieke Mulder and an aerial photograph of the field area showing the approximate locations (source: Google Earth)


3.3.1.2 Chemical characterisation, density fractionation and CNS Water repellency in soils is primarily attributed to the presence of hydrophobic organic compounds consisting of amphiphilic molecules from fresh or partly decomposed organic matter in the litter of vegetation (Van’t Woudt, 1959; Imeson et al., 1992; Doerr et al., 2000). Bulk properties of water repellency at the scale of soils are well understood, yet little remains known at the scale of individual soil particles. This existing hiatus justifies the study of the role of scale in water repellency. Thin layers consisting of several monolayers or many small adsorbed globules of organic compounds can coat particles and render it hydrophobic. Attempts to study the coating of soil particles using microscopes have produced inconclusive results and even escape the detection capacity of scanning electron microscopes (Doerr et al. 2000). It is hypothesized that hydrophobicity occurs in response to drying when drought stress influences the orientation of amphiphilic molecules present in the coatings of particles. Soils are rendered hydrophobic after substantial drying when the layer inverts, presenting the water-repellent tail of the amphiphilic molecules on the exterior, leading to an increase in the contact angle (Imeson et al. 1992, Dekker et al. 1998, Doerr et al., 2000). Soils are considered to be hydrophilic when molecules in the coating of the sand particles are oriented such that the polar ends of the molecules are ‘visible’ for water on the surface. The polar functional groups create a charged surface and make it possible for water to adhere to the particle without any problem. Several techniques have been used to understand the chemical composition of water repellent organic material in the field site samples. One of the newer techniques for extracting and understanding the chemistry of the samples was by means of a Density Fractionation (Cerli et al. 2011). There are basically three types of organic material the contribute to the carbon content of a soil and which can therefore contribute to soil water repellency and they are related to the arrangement of particles and interstitial matter (material between the pores of the sand particles). These three groups are recognised as either completely free, non-associative organic particles, those which are part of aggregates (conglomerations of organic matter and crystalline particles) and finally the material closely associated to the surface of the mineral. For extraction purposes and further analyses, one is always interested in extracting such compounds with minimal interference of chemical agents to leave the particles as intact as possible. For this effort a technique is used where a sodium polytungstate solution with an optimal separation density of 1.6 g/cm3 with inert chemical properties, neutral pH and recyclable properties provides an important basis (for the details on the procedure and the rationale for using a solution with these properties, we kindly refer to Cerli et al. 2011). A pre-determined quantity of soil sample is brought in suspension in the saline liquid and the material is left to sediment over a 1 hour period. Floating particles are sieved out, washed and dried and are referred to as the free Light fraction (f-L-fraction). Particulate organic material occluded in aggregates is commonly to heavy too float due to the binding with heavier and denser mineral particle. The supernatant is again put in suspension but now the aggregates are disintegrated using sonication (in an ultrasonic bath with the addition of 200J/ml). Now the new float is the organic material related to and occluded inside the aggregates (the O-fraction). Finally, the heavy fraction (H-fraction)) contains those organic compounds closely associated to the surface of the particles which influence the surface chemistry of the grains. Especially for studying the interactions between sand particles in microgravity, this latter fraction is of particular relevance. Subsequent measurements with a CN-analyser gives further insights in the development of the soil of the used samples. With the CN method, we measure the mass percentage of carbon by means of combustion. Grinded samples are heated to 1000°C and outgassing of the sample will yield CO2, H2O and N2. These compounds are measured using a gas-chromatograph and tested against a


reference column. By means of a computer tool we convert these read outs to mass percentages of carbon and nitrogen. Additional measurements (pending at the time of writing) will use more specific extraction techniques and GC-MS to further purify and quantify the chemical composition of the heavier fraction that covers the sand particles with a water repellent coating. 3.3.1.3 Pyrolysis and gas chromatography (GC-MS) As a test and proof of concept prior to the use of density fractionated samples in combination with GC-MS, we have tested the GC-MS technique independently to measure the chemical properties of various samples, in order to understand the effects of this technique and associated procedures on the sample material. In addition we assessed the effect of this technique on the obtained results. Gas-chromatography is widely used in soil chemistry to separate molecular components in a gaseous medium which facilitates their subsequent analysis. This technique is however insufficient when it comes to analysing long-chained molecules (such as those in water repellent coatings in soils) which have long retention times when intending to separate them on the basis of e.g. polarity. To study such processes gas chromatography - mass spectroscopy (GC-MS) is used in combination with pyrolysis where the sample material is heated to 700°C to break it up into separate fractions which can be analysed by the GC-MS on the basis of e.g. polarity (Wampler, 1999). Characteristic peaks in the chromatogram allow these components to be recognised and quantified. Reconstruction of the available components allows the soil scientist to determine which molecules were present in the first place using expert-knowledge and digital identification libraries. Organic material for these GC-MS analyses was extracted from soil samples by treating the samples with hydrogen fluoride (HF). HF dissolves the crystalline material and renders the samples with a high percentage of organic material. For this effort some 400 grams of the initial soil is exposed to 6ml 48% HF for two hours in plastic tubes. The supernatant was extracted after centrifugation and rinsed with HCl to remove residual traces of the HF and rinsed to obtain a neutral pH value of the sample. The obtained material was then analyzed with the GC-MS infrastructure in the soil science lab of the IBED Earth Surface Science group. 3.3.1.4 Physical characterization: sessile drop tests (SDT) The previous sections have outlined the procedures for characterizing the field conditions and the resulting chemical properties of the organic material which surround the individual sand particles. In the realm of soil physics, this chemistry comes together with the particulate nature of soils. Here, physical property of the contact angle of water reflects the interaction between the gas state, the liquid water and the solid state with the contributing effect of the particle coating. A method used to quantitatively measure the water repellency of a idealized layer of water repellent sand is with the use of the Sessile Drop Test (SDT). The technique itself is to date not widely employed in water repellency research in the domain of soil sciences, mostly because they yield results similar to the more common (and less technical) Water droplet penetration time (WDTP) and Molarity of Ethanol Droplet (MED) tests. Historically the SDT was used for the first time in relation to soils by Bachmann et al. (2000). Despite the prevalence of the other techniques, the SDT does provide a more quantitative measure for water repellency. Doerr & Dekker (2007) have a very positive outlook in this technique: “This elegant approach enabled the activation energy for the eventual wetting of a water repellent soil surface to be determined and allowed the authors to detect fundamental differences in the nature of water repellence between the two investigated locations. The [sessile drop] method introduced here has the potential to be an important tool in future work aiming to advance the understanding of the processes associated with changes between hydrophobic and wettable states of soils.”. On recommendation of the ESA Fly Your Thesis review


board and studies by the team, the SDT was eventually used to determine the physical bulk properties of the used sample material. The procedures for the SDT used for the ARID project was taken and modified from the work of Bachmann et al. (2000). A microscope slide is outfitted with a strip of double-sided tape (3M). Dehydrated sand particles of a known fraction of the soils were then glued to the tape and slightly pressed. Three droplets of water with a known size and volume are then placed on top of the layer with a calibrated Eppendorf pipit. A brief review of literature on these tests with soil samples learns that volumes vary between 0.20μl (Bachmann et al. 2010), 0.50μl (Bachmann et al. 2000) and 0.3μl (Diehl & Schaumann, 2007) and an average value of 0.20μl was thus used in all ARID related SDT measurements. The contact angle is then measured using digital still photography and digital analysis of the imagery. Usually these are measured using a conventional goniometer, but in order to prevent bias, we used a different set-up. A Leica macroscope was outfitted with a Digital Still Camera using ocular projection of the image and a special conversion tool that allowed the photography en-profile of the droplets. The conversion tool was developed for ARID and consists of a first surface mirror (glass disc with aluminium coating) under a 45 degree angle. Figure 11 shows the effect of this setup with a normal recording of the different sand particles used in the flight experiment (coated and non-coated) and a recording made for an SDT measurement. As a result of the heterogeneity of the sample material (irregular shaped particles by nature), a perfect contact angle measurement is not possible due to the lack of a flat surface (Bachmann & McHale, 2007). During each measurement two contact angles can be measured, although the interaction of the droplet with the substrate makes this not always possible, especially for larger fractions where the size of the particles can warp the shape of the droplet and locally forces an alteration in the actual contact angle. Nonetheless, the measurement of the contact angle is a more complex activity than commonly found in material sciences where the technique originated from (Ramírez-Flores, Bachmann & Marmur, 2010). Manual measurements are labour-intensive and prone to bias and hence we used a semi-automated algorithm for the measurement of the contact angle. Stalder et al. (2006) created a plug-in for the open-source image analysis programme ImageJ: Dropsnake1. Dropsnake offers two possible methods for calculating the contact angle and we employed the B-Spline Snake developed by Stalder et al. (2006) which uses a quadratic functions of splines which are fitted around the contours of the droplet (by polynomial fitting, see figure 11). The inputs of the user include the local angle of contact and the global shape of the droplet.

Figure 11 Images a sessile drop measurement using the Dropsnake toolbox to measure the water repellency of these particular grain-size of the soil sample under study (image: Jansen, 2010).

1 The Dropsnake toolbox is freely downloadable from http://bigwww.epfl.ch/demo/dropanalysis/ while ImageJ is downloadable for free via http://rsb.info.nih.gov/ij/


3.3.2 Sample variations during parabolic flight Three sample groups of soil particles (sand grains) were lab treated to make them hydrophilic, hydrophobic and non-coated. During collisions of particles, interactions by the presence or absence of electrostatics forces caused by of molecular interactions and polar moments will cause dissipation in energy that is quantifiable in terms of velocities before and after collisions, yielding a characteristic restitution factor. Each group will show collision behaviour unique to that group due to the lab treated conditions of the coating. Parameters such as particle size and initial velocity (tables of sample below) are crucial variables that influence particle collisions and interactions. As such, variations within each of the three sample groups cover these two parameters. Particle size will vary in groups from 100-500μm and sample location (different vegetation types) while velocities are varied in the range of 5-50mm/s. High resolution, high frame rate camera observation will provide a suitable temporal resolution for accurate particle image velocimetry. During the experiments, sufficient events for a statistical valid experiment will be recorded. The entire suit of variations will yield a dataset suitable for a mutual comparison between the three sample groups to determine characteristic interactions (restitution factors and energy dissipation) during collisions that are unique for each particular group, and understand their relation with the initial soil and vegetation conditions. For the flight were first established a preferred schedule (table 2) which was reiterated on site depending on the quality of the samples after transport. Since there are 3 separate test tubes on the platform,3 sets of data can be obtained. When listed ‘all 3’ this means that the material in the 3tubes is identical. Following the problems during the first flight, the sample schedule was changed to accommodate the samples that would produce the scientifically most interesting data (table 2) for our applications. Table 2 original sample schedule for the three flights.

FLIGHT DAY 1

Flight set Parabola Type in tubes Type Field Site Grainsize (μm)

1 0-5 All 3 sand 1 150-212 2 6-10 All 3 sand 2 150-212 3 11-15 All 3 sand 1 300-425 4 16-20 All 3 sand 2 300-425 5 21-25 All 3 sand 1 150-212 6 26-30 All 3 sand 2 150-212

FLIGHT DAY 2


1 0-5 All 3 sand 1 300-425 2 6-10 All 3 sand 2 300-425 3 11-15 All 3 sand/glass Lab <200, 300-400 4 16-20 Crystalline/fobic sand/glass Lab <200, 300-400 5 21-25 Crystalline/fobic glass Lab 300-400 6 26-30 Crystalline/fobic sand 3 106-150


- table 2 continued

FLIGHT DAY 3


1 0-5 All 3 sand 1 106-150 2 6-10 All 3 sand 2 106-150 3 11-15 All 3 sand 1 106-150 4 16-20 All 3 sand 2 106-150 5 21-25 All 3 sand 3 300-425 6 26-30 All 3 sand 3 300-425μm

Table 3 Sample schedule was re-evaluated during pre-flight preparation for the campaign. Programme shows the samples used during the three flights, as flown during the campaign in response to data flow problems (flight 1) and power outage during first set of parabola (flight 2). Since the samples on flight day 1 were the most interesting type, original schedule was changed in favour of the 212-300 series. Due to loss of data, these samples were re-flown in flight 3.

FLIGHT DAY 1

Flight set Parabola Type in tubes Type Field Site Grainsize (μm) status

1 0-5 All 3 Sand 1 212-300 Lost 2 6-10 All 3 Sand 2 212-300 Lost 3 11-15 All 3 Sand 1 300-425 Lost 4 16-20 All 3 Sand 2 300-425 Lost 5 21-25 All 3 Sand 1 212-300 Lost 6 26-30 All 3 Sand 2 212-300 Lost

FLIGHT DAY 2


1 0-5 All 3 Sand 1 300-425 Lost 2 6-10 All 3 Sand 2 300-425 Ok 3 11-15 All 3 sand/glass Lab <200, 300-400 Ok 4 16-20 Crystalline/fobic sand/glass Lab <200, 300-400 Ok 5 21-25 Crystalline/fobic glass Lab 300-400 Ok 6 26-30 All 3 sand 2 300-400 Ok

ok FLIGHT DAY 3


1 0-5 All 3 sand 1 212-300 Ok 2 6-10 All 3 sand 2 212-300 Ok 3 11-15 All 3 sand 1 300-425 Ok 4 16-20 Crystalline/fobic glass Lab 300-425 Ok 5 21-25 All 3 sand 1 212-300 Ok 6 26-30 All 3 sand 2 212-300 Ok


4. Parabolic Flight Campaign

4.1. Before the flights The week before the flight was dedicated to final hardware integration after transport. Activities included the installing of the base plates on the frame, the addition of padding and placement of the TERAS system. Much to the delight of both Novespace personnel and CEV (French test flight centre) the hardware was deemed fit for flight and quickly passed all incoming inspections without a problem. The hardware was uploaded to the aircraft and, after some required refitting, the experiment was the first FYT hardware onboard. Parallel to the hardware integration the TERAS system was checked out for its operational capabilities after transport from the Netherlands by car to Bordeaux and the system checked out fine. Inside the laboratory a bench was prepared for the preparation of the soil samples and the Leica macroscope was installed and outfitted with a camera for making the footage for the sessile drop measurement of the contact angles of the samples.

4.2. During and in-between the flights This section aims at giving an outline of the activities pre-flight, those procedures during the flight and the problems that arose during the operation of the experiment in microgravity. Especially the latter (sect. 4.2.2) aims at giving a purely factual overview of contingencies and steps taken to solve the problems. 4.2.1 Flight operations During the morning of each flight day, samples were transferred from their storage container to the polycarbonate flight containers. Packaged inside their anti-static zip lock bags, they were then transferred to the aircraft and stowed inside the sample drawer of the flight frame. At the same time, the TERAS system received a daily check-up (especially after the contingencies during FD1, see below). After take-off the TERAS system was powered-up and the LMS system was initialised. The configurations for the camera’s and library files were loaded for each set op parabola (5 consecutive parabolas, followed by longer breaks). During each flight a series of flat field and dark field were made to assist the image clean-up during the post-flight data processing. Before each parabola the settings for the LMS were selected in the interface to place the experiment ready for initialisation once the period of microgravity was started by the manoeuvre of the pilots. After each set of 5 parabolas a library file was closed and a new set was opened for the next parabola. During such breaks the two other operators were grounded to the flight frame when they opened the polycarbonate enclosure to (re)place samples on the platform. These procedures were repeated during each flight.


Figure 12 ARID flight crew ‘all smiles’ in microgravity. Photo’s Clockwise: the entire team flying and visible from left to right are: Christina Giannopapa, (journalist Erik Kuipers), Sebastiaan de Vet, (Fréderic Gai, Novespace) and Lieke Mulder. Second photo shows Christina and Sebastiaan working on the experiment in microgravity. Third photo show Lieke and Christina preparing TERAS for the next parabola. Photos courtesy of: ESA / Anneke le Floc’h. 4.2.2 In-flight problems and solution Flight 1, Tuesday 23rd May 2011 The first flight proved to be the most challenging flight of the three flight days due to the new experience of working in microgravity. Especially the operation of the camera system, concerning the activation and deactivation of the recording, proved to be a difficult activity in microgravity for the operators. This unforeseen contingency let to the decision to start each recording before the insertion of the parabola. While the TERAS system can accommodate the full 20 seconds in microgravity for an entire flight (total 30 minutes non-stop high-speed 500fps recording), the combined dataflow of starting at the pull-in and ending at the pull-out, caused a dataflow which caused the system’s hard disks to be completely full just half-way during the flight. Solution: A suitable solution was put in place in the procedures for the next flights and included the programming of short-cut keys which allowed the operator to control the recording directly with two keyboard commands. As a direct consequence of the full disk system, the post-flight data download took longer than anticipated and exceeded the available post-flight access inside the aircraft to the experiment. As a result the data download process (where the packets of the data are reconstructed and combined into one video file) needed to be aborted in the middle of the process. An (intermittently present) risk associated with such an action with the TERAS system and imaging software is that the register file of the recording’s library becomes corrupted and causes the data not to be accessible anymore. Such an error occurred after start-up early in the morning after the first flight while intending to continue downloading of the first flight’s data. With the data no longer retrievable we estimate a 90% loss of data of the first flight (meaning, 12 of the 15 recorded parabola, as the system was initially full after these 15 manoeuvres).


Solution: To prevent this type of contingency the system was reconfigured for the following flight with separate recording libraries for each series of 5 parabola. This proved to be a suitable solution and no problems arose during data download of the second and third flight. In the experiment protocol of the flights, a redundancy was already build in, so the loss of data was never catastrophic for the level, quality of amount of scientific data. The sample protocol was adjusted accordingly during the subsequent flights so that on flight 2 and 3 the experiment accommodated the sample types that were not recorded or lost during flight 1. Flight 2, Thursday 25th May 2011 Started off on a rough note when a power outage caused by an electrical problem of the experiment sharing the same electrical panel as the ARID experiment. This event caused a power fluctuation on the power bus (at least twice when the other experiment tried to replace fuses) and this fluctuation caused the main computer of TERAS to reboot. The interruption causes a corruption in the first library file due to the off-nominal interruption of the recording. Solution: The other experiment was subsequently shut down after to their outage to preserve the operational capabilities of the ARID experiment. In order to preserve the systems functionally during the remainder of the flight, all the disks of the TERAS system were quick-formatted and the system was reboot before the new recording. This solution proved to be excellent and the system worked without any problem during the remainder of the flight. Both data flow and the post-flight data reconstruction and download to the back-up disks took place without anomalies. Possibly some further research by the flight operators (Novespace & CEV) can address the power fluctuations on the power busses as these have shown to cause problems when our particular panel was shared between experiments. Overall, the flight produced 25 parabolas of data. Flight 3, Friday 26th May 2011 Posed no issues for the operation of the experiment, with a 100% flawless run of the modified experiment protocol selected for the last flight as a result of the preceding two days.


5. Scientific Results

5.1. Collected data during the Baseline Data Collection (BDC)

This section addresses the results obtained during the Baseline Data Collection of the experiment and those obtained during the parabolic flight campaign. Some of the used techniques explored for the BDC are focussed on the characterisation and extraction of chemical and physical properties for sand particles (not addressing the properties of interstitial matter) and have been a novelty for these types of applications in the soil science of water repellency research. Sections outlined below provide some discussion of these results and highlight their use in the future data processing activities.

5.1.1. Density fractionation, CN and GC-MS Two series of experiments and analyses have been carried out; those which were used to establish suitable field sites and properties, and those to better characterise the material in relation to the flight experiments. Below an integrative overview is given of these results to place the collected data in a framework for understanding their relevance for the coating properties of individual mineral soil particles. Table 4 Results of the density fractionation, showing the percentages of carbon and nitrogen in the organic material that contribute to the soil water repellency.

[ Confidential ] Following the density fractionation, the carbon an nitrogen values were measured using a CN (carbon/nitrogen) analyser. These results are given in table 4. In this table, site 1,2,3 are site with Dune moss with a closed cover, dune moss with an open cover and Dune shrub lands, respectively. The organic fractions of site 1 and 3 are comparable in their magnitude, yet they differ to a minor extend in their C/N ratios. On site 1 this may be indicative of more leaching of Nitrogen in soils with this vegetation cover. The data indicate that field site 1 is more capable of producing aggregates, possibly attributed to better growing vegetation and/or the conditions inside the soil such as the height of the water table. However, the C/N rations of the f-l-fraction and O-fraction are similar and quite low and indicate little development inside the soil which drives the idea that aggregates inside these soils are weak and lack the cohesive strength to stay together. In addition to these CN analyses of the density fractionations, CN measurements were also carried out per individual size fraction to get a better indication of the occurrence of organic material inside the distribution of soil. These data are included in table 5.


Table 5 - Percentage of carbon measured per size fraction using a carbon/nitrogen/sulfur analysis (CNS/Dumas) expressed in (kg x kg-1) x 100 (data: J.Jansen, 2010).

[ Confidential ] In combination with microscopic observations of these fractions, it has become clear how the majority of the organic material has been occluded in aggregates inside the soil. Experiments with hydrogen peroxide treatments and hydrochloric acid (HCl) followed by sieving systematically yields smaller fractions than when aggregates were intact. Especially in larger fractions 600-850 μm the aggregation was mostly dominant as shown by the aggregation in figure 14. Hence care was given in the treatment and selection of flight samples to exclude these properties by selecting smaller size-ranges where these properties were not present or at least reduced to a minimum.

Figure 14 microscope image showing aggregation of soil particles due to organic material into larger conglomerates. The particle sizes in the ARID experiment were deliberately selected to be below the size (600-850μm) where this effect was most frequently observed. A more detailed chemical analyses was for the BDC was the GC-MS of soil samples from the Dutch dunes. Here we calculated the ration between the A-polar and Polar segments as a measure for the water repellency. During the pyrolysis of the samples, the molecules are broken up into smaller pieces which originally consisted of molecules with a polar functional group and an a-polar tail. The A/P ration thus gives a ration for the average length of the amphiphilic molecule that coated the sand particles (assuming that all particles were part of an amphiphilic molecule in the first place as we selected the sample material for these properties). In other words, the larger the A/P ration, the longer the molecule and the more water repellent the soil particle. Results obtained by the GC-MS analyses are given in table 7 below.


Table 6 Overview of the various properties measured using GC-MS and the associated measurements of the field conditions and samples. Reference sites #1-5 correspond to the sites listed in figure 10. (data: L. de Graaf, 2010).

[ Confidential ] Visible in the table (6) is the interrelationship between the A/P ratio’s of the soil samples, water repellency parameters and the negatively correlated relation with the vegetation. Contrary to the liquid used in the density fractionation, there is little consensus in literature about the influence of HF on organic material. During future additional GC-MS analyses we will first extract the organic material, followed by the actual GC-MS extraction.

5.1.2. Physical characteristics During the flight campaign the sessile drops were also carried out for the flight sample material. These measurements served as a quality control to assess the state of the sample material prior to and after transportation towards the campaign locations. Results of the baseline sessile drop tests (not those measurements of the parabolic flight experiment) presented here are of a the similar field site (location 2 in tables 6 and 7) to the flight samples. Table 7 Overview of the Pearson’s r correlation coefficient matrix of various contact angle (CA) measurements in series, grain sizes and % of carbon. Values close to 1 or -1 are positive correlations. Note that in this matrix only linear correlations are tested and not individual grain size fraction. (data: Jansen 2010).

[ Confidential ] These measurements allowed us to make a sensitivity analysis of the technique, and one of the major outcomes is the way in which the material is processed and order of measurements affects the correlations between the various soil parameters displayed in table 8. These lessons were incorporated in a revised protocol for the flight samples where the exposure to the ambient air humidity. When considering a more homogeneous distribution across all the factions, the assumption is made that until the pore space of the soil reaches the capillary length of water (273 μm), water repellency increases with grain size. After all, in the smaller size fractions the organic material can adhere to a larger surface area compared to the larger grain sizes. We observe that such an idealised assumption is not possible as the carbon is not evenly distributed across the samples, and contact angles (strength of water repellency) of the smaller fraction are larger. We can see that (table 6) the carbon is the highest in the smallest fractions (<106μm) from the TOC (total organic carbon) measurements and the majority of this sample material is darker in colour are a result of free organic particulate matter. And the valid assumption can be made that this interstitial matter influences the hydrophobicity of the material (analogue to the findings of Doerr et al., 2005). In coarser fractions (>106μm) free organic particles were not observed, although above 600μm the organic material was occluded in aggregates.


Figure 15 Images of two different samples used in the experiment; the left image shows sand particles coated with a water-repellent layer (this is the type of organic material that is associated to the H-fraction of the density fractionation) and sand particles bleached with H2O2 and HCl to remove the organic coating (images: Jansen, 2010). 5.1.3 Acceleration data Acceleration data is available in the format of Novespace where the files were distributed after each flight and during the first flight the accelerations across 3 axis were recorded with the accelerometer of the ARID set-up. As a result of power loss of the battery of the unit, no high-resolution acceleration data of this accelerometer was available for the third flight.

Figure 16 Example of the data of a parabola on flight day 2 provided by Novespace.


5.1.4 Temperature measurements Temperature measurements from inside the polycarbonate enclosure and show the daily diurnal cycle, the opening and cooling of the aircraft in the morning by air-conditioning and the rising temperature inside the enclosure by several degrees as a result of the electrical equipment (no temperature rise was encountered that negatively affected the soil samples), and even the cooling during the opening of the enclosure for the change of the batch of samples. The recorded temperature range and duration was well below the conditions where the temperature can influence the coating conditions of the sand particles.

Figure 17 Selection of the temperature during flight day 2, showing the ambient temperture during night-time, followed by the openeing and cooling of the aricraft interiour and the heating as aresult of the diurnal cycle, in combination with the heating of electricalequipment inside the enclosure.

5.2. Data processing and analysis At the time of writing this document, data processing is still taking place and no results of the parabolic flight experiment can be presented, as yet. Due to the large bulk of data, an automated approach should be undertaken, for which expertise is required which is not available at the University of Amsterdam. Hence, the team requires the support in data processing of related expertise from the mathematics/physics department of the TU Eindhoven (TUe). Regrettably, during the first semester of the 2011 curriculum, no MSc/PhD candidate was found to be available for assisting in the processing of the data. Meanwhile, with the start of the new semester in February 2012 a new call will be issued for an MSc/PhD project to support the processing of the data. Since the MSc theses typically to no start in the first semester, changes of finding candidates are high. We are also exploring other options for a more speedy data processing. For giving an indication what the data processing encompasses, the sections below address more specific points. 5.2.1 Background of particle interactions The approach of the proposed project utilises theoretical concepts of colloid physics and electrostatic self-assembly where relevant mechanisms to this problem are rooted (e.g. Hurd, 1995;


Okubo & Tshuchida, 2002; Grzybowksi et al. 2003). The presence of a surface charge created by the orientation of the coating provides an important basis for the study of hydrophobicity mechanisms. In this study, particles suspended in air bear a physical resemblance to other colloidal studies where interactions between particles with charged surfaces suspended in a medium are studied (although these charge difference are usually formed by the suspension of particles in polar liquids). It is important to distinguish that the surface charge, which is the system under consideration, is only present when the coating is hydrophilic and hence it is absent when particles are rendered hydrophobic by drying. The interaction between particles of these two characteristic groups makes it possible to understand charge effects caused by coatings and allows a comparison for determining characteristics and mechanisms for individual particles. Scale effects are important to consider as macroscale particles in the order of 100-200μm will generally experience more influence from gravity, collisions and friction, whereas microscale particles (<50μm) are mainly influenced by London- Van der Waals (L-vdW) forces and long range electromagnetic forces (Aranson and Tsimring, 2006). Ultimately, interactions between particles depend on the interparticle distance. Within a few-nm range (r-12 with r denoting the intermolecular distance) all molecules will suffer from Born-repulsion and tend to repel each other due to overlapping outer orbits of their electron shell. On the scale of tens of nm a transition occurs from repulsion to the attraction of the L-vdW force (r-6) (Velarde, 2008). The proximity of two charged surfaces determines the interaction. Asperities at the surface of particles can be considered to range in size from 30-60nm for smooth particles to 200-300nm for rough particles (Pholman et al. 2006). Such asperities prevent molecules in the coatings from experiencing Born-repulsion in close contact. The actual proximity of these interactions is usually defined by the thickness of the electric double layer created by surface charge. This Deybe length (Dl) is defined as:

(1)

Where e is the charge, ε is the dielectric constant of the medium, kB the Boltzmann constant and n the amount of cations. The largest length observed in water is in the range of 1μm (Okubo et al. 2000) while long-range interactions for interfacial systems can overcome many Deybe-lengths even with interactions trough the air. Studies have shown strong interactions for such systems across distances of 10μm (Hurd, 1985). Based on scale, surface characteristics and the properties of the sand particle’s coatings, particles will experience the strongest interaction across larger distances from the dipoles formed by the polar functional groups of the amphiphilic molecules in the coating. Quantifying the interactions between these particles can be based on the collisions between particles. The energy of the system plays an important role in determining the coating properties during these collisions. Thermodynamic laws on conservation of energy dictate that the internal energy of the system remains constant while interactions between coatings will cause a change in energy of individual components. Energy dissipation during collision has been a well studied phenomenon (e.g. Brillantov and Pöschel, 2004). The relation between the observed velocities before (v1,2) and after the collision (v’1,2) for two (nearly) identical particles is given by:

(2)

Where n12 is a unit vector oriented at the moment of the collision from the centre of particle 1 to the centre of particle 2. A restitution coefficient e defines the fraction of energy lost during the collision


and varies for inelastic collisions (e=0) and elastic collisions (e=1). The total energy loss ΔE during the collision is defined for 0<e<1 in the form of:

(3)

The restitution coefficient is an important parameter that not only depends on the relative velocity of colliding particles or difference in mass between particles but also the point of contact is relevant for non-spherical particles such as sand (Goldsmith, 1964). Electrostatic interactions between coatings localised on point(s) of contact will affect the restitution coefficient e that is quantifiable by the change in velocities before (v1,2) and after the collision (v’1,2). A large number of collision events will need to be recorded in order to statistically account for size distributions of samples and variations in shape. The large coated surface area creates ample locations where particles are capable of interacting. The quantification of minute changes in e require high-accuracy observations of collision parameters with a high temporal resolutions using particle image velocimetry (e.g. Dahl et al. 1995; Lee and Bryanston-Cross, 1997; Prasad, 2000) and can set a magnitude and scale for these interactions. By differentiating between groups with various coating properties (hydrophilic, hydrophobic and non-coated particles) it is possible to derive relevant characteristics properties and strengths of the water repellent coatings. While coatings have an identical composition, hydrophilic oriented coatings will show a dissipation of energy ΔE and therefore changes denoted in terms of v1,2 , v’1,2 ,e due to the interactions between the polar functional groups presented on the exterior of the coating. Hydrophobic (water repellent) coatings and non-coated particles on the other hand will show minor interaction, as the magnitude of interactions is considerably lower as these lack the strong interactions of polar functional groups. Differences in dissipation allow the determination of coating orientations and mechanisms to determine these scale effects.

Figure 18 Inflight photo showing the screen with the camera views inside the experiment. Imagery obatined with the two camera’s will be used to reconstruct the collisions of particles in 3D, to gain the best vectors for determning the collision dynamics and properties. Image courtesy: ESA/Anneke le Floc’h.


5.3. Outlook and application of experimental results Soil water repellency is a growing problem in many European countries within the context of our changing global climate. This environmental challenge has a deeply-rooted societal component. In the Mediterranean areas the combination of changing precipitation patterns resulting from climate change lead to higher drought stress and intensified precipitation. During torrent-events agricultural areas are easily eroded by mudflows and other forms of erosion when the large amount of water cannot infiltrate the soil. This increased overland flow of water also increases natural hazards for nature, wildlife and the local population. Soil water repellency does not only affect Mediterranean areas in Spain, France, Italy and Greece, but it is common too in countries with temperate climates such as The Netherlands and Denmark. Increased erosion resulting from water repellency in coastal defence landforms such as the Dutch dune systems are of particular relevance for the security and livelihood in these countries. Means for combating and mitigating water repellency are being studied world-wide. Understanding the broad-scale dimension of water repellency allows more effective strategies to be developed. The results of the ARID experiment fit well within the larger context and contribute to the linking and understanding of large scale effects of water repellency in relation to the small micro-scale of the coating chemistry. New insights gained by the application of the microgravity experiment data can improve our understanding of soil water repellency and may influence the development of better mitigation techniques and land management strategies. Apart from terrestrial applications of particle dynamics, other studies closely related to the experimental approach in this study (Poppe et al., 2002; Heißelmann et al. 2009) have shown effects of collisions in microgravity and illustrate the use of the determination of restitution coefficients of collisions between individual particles. Based on such studies, the studied interactions in this project (although not within the current scope) may also be usable for applications in the study of dust particle interactions in protoplanetary disks. These disks found throughout the Milky Way are the locations where gas and dust aggregate to form the infantile versions of solar systems. Inside these dust disks, interactions and collisions cause particles to aggregate and when sufficiently large, gravity takes over to form them into planetesimals, the building block for planets and asteroids. Dynamics and aggregation of sub-millimetre to millimetre sized particles as a result of collisions are important to understand the depletion of gas and dust at various timescale in stellar and planetary evolution. Other locations where these particles interactions are of relevance are for example the rings of Saturn (Heißelmann et al. 2009). While waiver-thin in the order of a few hundred meters, the rings consist of many small ice and dust particles that reflect the light and make the rings visible to be seen from Earth. Here too particle interactions dictate the formation of the Saturnian ring system.


6. (Preliminary) Conclusions

[ Confidential ]


7. Acknowledgements The ARID team is grateful for the unique opportunity given by the ESA Education Office (X. ..., N.Callens, E. Zornoza) of flying this experiment in microgravity onboard the 54th ESA parabolic Flight Campaign in Bordeaux and the (financial and communication) support offered by The Netherlands Space Office (NSO) and the Earth Surface Science group of the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the University of Amsterdam. Much of the efforts taken for this project would not have been possible without the endorsement and enthusiastic support of prof. dr. K. Kalbitz and dr. E. Cammeraat and the contributions of Novespace (especially our technical interface, F. Gai), the Quick Service engineering workshop of the University of Amsterdam (W.Aartsen), University of Hannover (J. Bachmann and M-O Goebel), the Life & Physical Sciences Instrumentation and Life Support Section of ESA ESTEC (R. Lindner, A. Dowson and Z. Ilievksi) for allocating the TERAS system and the flexibility with the loan of the system after the flight’s postponement early 211, our lab specialists (C.Cerli, X. Wang, L. Hoitinga), the supporting BSc students (L. de Graaf, J.Jansen, S. Liem, R. van Hall), D. Kosterman for welding activities on the transport base of the flight frame, our corporate sponsors Bradford Engineering (E. van der Kroon), Elektro ABI, RIMAS, Labstox, Brüel & Kjær Benelux, and those persons whom we may have forgotten to mention here. 8. References

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